T1 decay for simulator (#4326)

When using a simulator, noise is applied after each gate. Previously, in the t1 decay method, there's only one wait gate, which means that noise is not properly applied "over time" when using a simulator. I've updated the method to add a number of wait gates proportional to the length of the delay when using a simulator.

Related to Issue #[4264](#4264)

As always, any feedback is appreciated!

cirq-core/cirq/experiments/t1_decay_experiment.py

@@ -67,7 +67,10 @@ def t1_decay(
     var = sympy.Symbol('delay_ns')
 
     sweep = study.Linspace(
-        var, start=min_delay_dur.total_nanos(), stop=max_delay_dur.total_nanos(), length=num_points
+        var,
+        start=min_delay_dur.total_nanos(),
+        stop=max_delay_dur.total_nanos(),
+        length=num_points,
     )
 
     circuit = circuits.Circuit(

cirq-core/cirq/experiments/t1_decay_experiment_test.py

@@ -167,32 +167,6 @@ def test_all_off_results():
     )
 
 
-@pytest.mark.parametrize(
-    't1, data',
-    [
-        (100, [[100.0, 6, 4], [400.0, 10, 0], [700.0, 10, 0], [1000.0, 10, 0]]),
-        (400, [[100.0, 0, 10], [400.0, 6, 4], [700.0, 10, 0], [1000.0, 10, 0]]),
-        (
-            200,
-            [
-                [time, int(np.exp(-time / 200)), 10 - int(np.exp(-time / 200))]
-                for time in np.linspace(0, 1000, 100)
-            ],
-        ),
-    ],
-)
-def test_constant(t1, data):
-    result = cirq.experiments.T1DecayResult(
-        data=pd.DataFrame(
-            columns=['delay_ns', 'false_count', 'true_count'],
-            index=range(len(data)),
-            data=data,
-        )
-    )
-
-    assert np.isclose(result.constant, t1, 5)
-
-
 def test_curve_fit_plot_works():
     good_fit = cirq.experiments.T1DecayResult(
         data=pd.DataFrame(
@@ -228,6 +202,60 @@ def test_curve_fit_plot_warning():
         bad_fit.plot(include_fit=True)
 
 
+@pytest.mark.parametrize('t1', [200, 500, 700])
+def test_noise_model_continous(t1):
+    class GradualDecay(cirq.NoiseModel):
+        def __init__(self, t1: float):
+            self.t1 = t1
+
+        def noisy_moment(self, moment, system_qubits):
+            duration = max(
+                (
+                    op.gate.duration
+                    for op in moment.operations
+                    if isinstance(op.gate, cirq.WaitGate)
+                ),
+                default=cirq.Duration(nanos=0),
+            )
+            if duration > cirq.Duration(nanos=0):
+                # Found a wait gate in this moment.
+                return cirq.amplitude_damp(1 - np.exp(-duration.total_nanos() / self.t1)).on_each(
+                    system_qubits
+                )
+            return moment
+
+    results = cirq.experiments.t1_decay(
+        sampler=cirq.DensityMatrixSimulator(noise=GradualDecay(t1)),
+        qubit=cirq.GridQubit(0, 0),
+        num_points=4,
+        repetitions=10,
+        min_delay=cirq.Duration(nanos=100),
+        max_delay=cirq.Duration(micros=1),
+    )
+
+    assert np.isclose(results.constant, t1, 50)
+
+
+@pytest.mark.parametrize('gamma', [0.01, 0.05, 0.1])
+def test_noise_model_discrete(gamma):
+    results = cirq.experiments.t1_decay(
+        sampler=cirq.DensityMatrixSimulator(
+            noise=cirq.NoiseModel.from_noise_model_like(cirq.amplitude_damp(gamma))
+        ),
+        qubit=cirq.GridQubit(0, 0),
+        num_points=4,
+        repetitions=100,
+        min_delay=cirq.Duration(nanos=100),
+        max_delay=cirq.Duration(micros=1),
+    )
+
+    data = results.data
+    probs = data['true_count'] / (data['true_count'] + data['false_count'])
+
+    # Check that there is no decay in probability over time
+    np.testing.assert_allclose(probs, np.mean(probs), atol=0.2)
+
+
 def test_bad_args():
     with pytest.raises(ValueError, match='repetitions <= 0'):
         _ = cirq.experiments.t1_decay(